Air traffic flow management (ATFM) is the regulation of air traffic in order to avoid exceeding airport, airspace, or air traffic control capacity in handling traffic, and to ensure that available capacity is used efficiently. Because only one aircraft can land or depart from a runway at a given time, and because aircraft must be separated by time to avoid collisions, every airport has a finite capacity (i.e., airports can safely handle only so many aircraft per hour). This capacity depends on many factors, such as the number of runways available, layout of taxi tracks, availability of air traffic control, and current or anticipated weather. Weather can cause large variations in capacity because strong winds may limit the number of runways available, and poor visibility may necessitate increases in separation between aircraft. Similarly, there are limits on the number of aircraft that can flow through navigational fixes in the en-route airspace, as well as limits on the number of aircraft that can exist in a given region of airspace (called sectors) at any given time. The latter is driven by limits on the number of aircraft an air traffic controller can safely handle simultaneously. Air traffic control can also be limiting, there are only so many aircraft an air traffic control unit can safely handle. Staff shortages, radar maintenance or equipment faults can lower capacity.
When capacity of a resource becomes insufficient (for example, due to weather or poor planning), aircraft may be directed towards holding patterns where they circle until it is their turn to land or are instructed to wait on ground. But aircraft flying in circles and waiting on the tarmac of an airport are inefficient and costly ways of delaying aircraft. It is more efficient to plan and account for uncertainty in capacity so that resources are fully utilized without (or with lower probability of) exceeding capacity.